The Synthesist: Climate Change Could Amp Up Ocean Noise
In recent honor of World Oceans Day, and what would have been Jacques Cousteau’s 100th birthday, and the thirty-fifth anniversary of “Jaws,” and not least because it was hot as all global warming outside, I sought refuge in “Oceans,” the majestic new documentary film by the directors of “Winged Migration” and “Microcosmos.” Only one theater within 300 miles was showing it, at only one showtime, and I caught it on what turned out to be the last day. As I sat all but alone in the fading light, my timing felt ominously apt. I’m always struck by how quiet ocean documentaries are. One hears the ghostly soundings of whales, of course, the eager pip of dolphins, the clacking of a crab’s claws. But invariably, as if to cover for an awkward and extended natural silence, the soundtrack swoops in, alternately dreamy and orchestral, with strings or harps or xylophones. In 1953 Cousteau called the ocean “the silent world”; six decades later, despite otherwise profound advances in cinematography, one would be forgiven for thinking he was right. But in that time science has come to understand that the sea brims with biological sound. Many if not most fish, for instance, communicate audibly. The croaker, the sea robin, and the sea trout seek mates and frighten enemies with honks and gurgles they produce with their swim bladders. The parrotfish, the garibaldi, the bar jack, and the scad grind their teeth with a rasping sound (think nails on chalkboard) to ward off intruders. The toadfish hoots like an owl, the cowfish barks like a dog; herring fart. The northern seahorse, in courtship, flicks a protuberance on its bony skull that clicks and snaps like a castanet. The black drum croaks so loudly in the canals of Florida that it can be heard through the ground and into the homes of nearby residents. Blame our senses for the oversight. Slaves to light, we forget that the vast majority of sea life resides in darkness, below the photic zone, and can’t rely on visual cues. And sound behaves differently underwater. Although high-frequency noise quickly attenuates, as the water absorbs it, low-frequency sounds, especially those between 10 to 200 Hz (roughly the range of bass guitar; humans can hear frequencies up to 20,000 Hz), can travel far, even miles. Sometimes BBC video crews ask Stephen Simpson, a reef biologist at the University of Bristol, to borrow some reef sounds for a film they’re making. He regrets to tell them that he has no stereo recordings, which can be played through two separate audio channels and sound great in a movie theater; sound travels too fast under water — five times faster than in air — to be audible in anything but mono. “It’s an alien acoustic environment,” Simpson says, “one we may not be ready for, I guess.” Coral reefs are particularly loud. Some noise comes from the wind and breaking waves, but most is low-frequency fish chatter and the collective claw-clicking of snapping shrimp, which sounds “like heavy rain on a tin roof,” Simpson says, and be can loud enough to impede the use of military sonar. Put it all together and you get “a pretty complex soundscape,” he notes. Indeed, Simpson has found that different reef habitats—barrier or fringing, mangrove or sandflat, pristine or degraded by sediment or overfishing — have different, identifiable audio signatures. Biologists traditionally study the health and diversity of reefs through visual surveys, which have their limitations — night, for instance. Lately Simpson has begun thinking that reef noise could be a useful monitoring tool. One could record several reefs in a day, for a quick overview, or leave the recording equipment out for months to collect long-term data. But reef noise isn’t a mere byproduct; it is instrumental in the reefs’ very formation, Simpson has found. Biologists have long wondered how young reef fish, which are cast into the open ocean as tiny larvae, manage to find their home reef days or weeks later. In a neat experiment a few years ago, Simpson set up two light traps, one quiet and the other crackling with the piped-in sounds reef fish and crustaceans. Larval fish were clearly more drawn to the latter. The reef noise is a homing beacon, Simpson says, “a roadmap that these organisms use to find their way.” In fact, reef fish are sensitive to sound even as embryos and become more sensitive to it, and in wider range of frequencies, as they develop. Nor is the phenomenon restricted to fish. Recently Simpson and some colleagues working in the Caribbean found that baby corals — mere flea-sized sacks of cells — orient by sound before settling into a hardened station on the reef. They can choose a direction and go. “When the idea was first suggested, I thought it was pretty out there,” Simpson concedes. “Look at it: it’s a blob covered in hair cells, it doesn’t have a central nervous system, an auditory apparatus, or anything.” But, he notes, those hair cells, or cilia, are akin to those in our own inner ear, where, when waggled by vibrating particles, they help detect sound. The cilia on coral larvae my serve the same purpose; they may even be tuned to specific frequencies. In effect, every larva is an inside-out ear; the reef literally broadcasts itself into existence. “Our instinct had been to assume that they’re pretty much pathetic,” Simpson says of the larvae. “But the more we learn, the more amazed we become. They can hear, smell, pick their habitats — they have control over their destiny.” Up to a point, sadly. Even as scientists expand their appreciation for the sea’s natural sounds, they have grown troubled by the rising tide of human-made noise. The deafening effect of seismic and sonar blasts on dolphins and whales is well documented. But low-frequency noise has become more pervasive too, especially near shore: more shipping traffic, more recreational boating, more underwater pile-driving. One recent study off the California coast found that underwater sound levels at the lowest frequencies have increased by an order of magnitude since the 1960s. How this aural fog might effect sea life is unclear. Could a rise in noise change where and how fish school, as traffic noise alters the flocking and nesting behavior of birds? Might it mask their ability to communicate, reproduce, seek prey and avoid predators — or, in the case of young corals, find their home reefs and build upon them? Or might they learn to hear around it, they way we acclimate to a noisy air-conditioner or the background din of a cocktail party. “When it comes to the chronic effects of sound,” Simpson says, “nobody has any real solid evidence.” Climate change will only raise the volume. As the ocean warms, it will become more acidic; that hinders animals like corals from forming carbonate shells. It also reduces the concentration of sound-absorbing chemicals like boric acid and magnesium sulphate, enabling low-frequency noise to travel farther, according to a recent study led by Tatiana Ilyina of the University of Hawaii at Honolulu. “The ocean,” Ilyina writes, “is becoming transparent to sound.” If that’s not enough, the ears of fishes may change too. Fishes rely on the otolith, a carbonate structure in the ear, to orient themselves and sense their surroundings. But a study last year from the Scripps Institution of Oceanography found that white sea bass reared in carbon-dioxide-rich water grow otoliths that are bigger than expected. It’s not yet clear if the size difference affects function, but symmetry does. In a separate study, Simpson recently found that reef fishes with asymmetrical otoliths have a harder time hearing the preferred sounds of their reef. The fish face a kind of tinnitus, from without and within. The seas are in trouble. One need only watch a few minutes of video of the Deepwater oil disaster, streaming live courtesy of a remotely operated submersible vehicle, to grasp the scale of the harm we wreak. Through the artistry of film, we can marvel at the sea life that whose grace and beauty we may soon forever miss. The shame — one of many — is we may never truly hear it.
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The Synthesist: Climate Change Could Amp Up Ocean Noise